1. Field of the Invention
The invention relates to an improved mixing device for use in biological actor apparatus. More specifically, this invention relates to an improved float tube assembly for a sludge mixing apparatus for use in digesting apparatus for treating municipal waste sludge and the like.
Typically, digesting apparatus for treating municipal sludge waste comprises a tank having an inlet and outlet and a floating or fixed cover or dome. Gas evolving material, commonly referred to as sludge, to be anaerobically digested is admitted through the sludge feed pipe (inlet) to the digester, while supernant or treated material is withdrawn through the outlet located at the desired surface level of the liquid.
2. State of the Art
During the anaerobic digestion of the sludge gas evolves and is trapped under the cover of the digester. Since the rate of gas production varies it is desirable to use floating ballast type covers for the digester. Such covers are shown in U.S. Pat. Nos. 3,535,236, 4,378,437 and 4,391,705.
To enhance the rate of the digestion of the organic material in the sludge, it is desirable to mix the contents of the digester. Typical prior art mixing apparatus for use in digesters are shown in U.S. Pat. Nos. 4,422,771, 4,575,256 and 4,997,557. To promote efficient mixing of the sludge in the digester it is preferred that the liquid, scum, and any foam thereon be continuously mixed into the materials in the lower part of the digester. Since the fluid level in the digester can vary as well as the amount of free liquid, scum and any foam thereon, even though the digester may include a floating cover, accurate vertical placement of the mixing apparatus within the digester is difficult to ensure so that at any point in time the mixing apparatus inlet will have access to free liquid, scum and any foam thereon for mixing into the material therebelow. If the mixing apparatus does not have access to liquid to mix into the sludge, little beneficial mixing of the sludge can take place. It is also desirable that the mixing apparatus have access to any scum and foam on the surface of the liquid in the digester so that the mixing apparatus can break down the scum and foam.
Various types of floating inlet and skimming apparatus for digesters are illustrated in U.S. Pat. Nos. 3,303,932, 4,094,338, 4,153,071, 4,024,062 and 4,956,100. Other types of skimming apparatus are illustrated in U.S. Pat. Nos. 3,633,749, 4,663,037, 4,761,225, 4,626,358 and 4,998,585.